148 research outputs found
Is brane cosmology predictable?
The creation of the inflationary brane universe in 5d bulk Einstein and
Einstein-Gauss-Bonnet gravity is considered. We demonstrate that emerging
universe is ambigious due to arbitrary function dependence of the junction
conditions (or freedom in the choice of boundary terms). We argue that some
fundamental physical principle (which may be related with AdS/CFT
correspondence) is necessary in order to fix the 4d geometry in unique way.Comment: LaTeX file, 4 page
A Field Range Bound for General Single-Field Inflation
We explore the consequences of a detection of primordial tensor fluctuations
for general single-field models of inflation. Using the effective theory of
inflation, we propose a generalization of the Lyth bound. Our bound applies to
all single-field models with two-derivative kinetic terms for the scalar
fluctuations and is always stronger than the corresponding bound for slow-roll
models. This shows that non-trivial dynamics can't evade the Lyth bound. We
also present a weaker, but completely universal bound that holds whenever the
Null Energy Condition (NEC) is satisfied at horizon crossing.Comment: 16 page
Revisiting the calculations of inflationary perturbations
We present a new approximation scheme that allows us to increase the accuracy
of analytical predictions of the power spectra of inflationary perturbations
for two specific classes of inflationary models. Among these models are chaotic
inflation with a monomial potential, power-law inflation and natural inflation
(inflation at a maximum). After reviewing the established first order results
we calculate the amplitudes and spectral indices for these classes of models at
higher orders in the slow-roll parameters for scalar and tensorial
perturbations.Comment: Extended version of the talk to be published in the proceedings of
the Mexican Meeting on Exact Solutions and Scalar Fields in Gravity. Mexico,
1-6 October, 200
Power-Law Inflation from the Rolling Tachyon
Modeling the potential by an inverse square law in terms of the tachyon field
() we find exact solution for spatially flat isotropic
universe.We show that for the model undergoes power-law
inflation. A way to construct other exact solutions is specified and
exemplified.Comment: References added. Matches the version in print. To appear in PR
Oscillatory behavior of closed isotropic models in second order gravity theory
Homogeneous and isotropic models are studied in the Jordan frame of the
second order gravity theory. The late time evolution of the models is analysed
with the methods of the dynamical systems. The normal form of the dynamical
system has periodic solutions for a large set of initial conditions. This
implies that an initially expanding closed isotropic universe may exhibit
oscillatory behaviour.Comment: 16 pages, 3 figures. With some minor improvements. To appear in
General Relativity and Gravitatio
CMB Power Spectrum from Noncommutative Spacetime
Very recent CMB data of WMAP offers an opportunity to test inflation models,
in particular, the running of spectral index is quite new and can be used to
rule out some models. We show that an noncommutative spacetime inflation model
gives a good explanation of these new results. In fitting the data, we also
obtain a relationship between the noncommutative parameter (string scale) and
the ending time of inflation.Comment: 8 pages, 2 figures; v2: refs. added and minor corrections; v3:
further minor correctio
Consistency relation for the Lorentz invariant single-field inflation
In this paper we compute the sizes of equilateral and orthogonal shape
bispectrum for the general Lorentz invariant single-field inflation. The
stability of field theory implies a non-negative square of sound speed which
leads to a consistency relation between the sizes of orthogonal and equilateral
shape bispectrum, namely . In
particular, for the single-field Dirac-Born-Infeld (DBI) inflation, the
consistency relation becomes . These consistency relations are also valid in the
mixed scenario where the quantum fluctuations of some other light scalar fields
contribute to a part of total curvature perturbation on the super-horizon scale
and may generate a local form bispectrum. A distinguishing prediction of the
mixed scenario is . Comparing
these consistency relations to WMAP 7yr data, there is still a big room for the
Lorentz invariant inflation, but DBI inflation has been disfavored at more than
68% CL.Comment: 4 pages, 2 figures; v2: title changed, some mistakes corrected; v3:
refs added, version accepted for publication in JCA
On the thin-shell limit of branes in the presence of Gauss-Bonnet interactions
In this paper we study thick-shell braneworld models in the presence of a
Gauss-Bonnet term. We discuss the peculiarities of the attainment of the
thin-shell limit in this case and compare them with the same situation in
Einstein gravity. We describe the two simplest families of thick-brane models
(parametrized by the shell thickness) one can think of. In the thin-shell
limit, one family is characterized by the constancy of its internal density
profile (a simple structure for the matter sector) and the other by the
constancy of its internal curvature scalar (a simple structure for the
geometric sector). We find that these two families are actually equivalent in
Einstein gravity and that the presence of the Gauss-Bonnet term breaks this
equivalence. In the second case, a shell will always keep some non-trivial
internal structure, either on the matter or on the geometric sectors, even in
the thin-shell limit.Comment: 17 pages, 2 figures, RevTeX 4. Revised version accepted for
publication in Physical Review
Modern cosmologies from empty Kaluza-Klein solutions in 5D
We show that the empty five-dimensional solutions of
Davidson-Sonnenschtein-Vozmediano, {\em Phys. Rev.} {\bf D32} (1985)1330, in
the "old" Kaluza-Klein gravity, under appropriate interpretation can generate
an ample variety of cosmological models in 4D, which include the
higher-dimensional modifications to general relativity predicted by "modern"
versions of noncompactified 5D gravity as, e.g., induced-matter and braneworld
theories. This is the first time that these solutions are investigated in a
systematic way as embeddings for cosmological models in 4D. They provide a
different formulation, which is complementary to the approaches used in current
versions of 5D relativity.Comment: Accepted for publication in JHE
Inflationary cosmology in the central region of String/M-theory moduli space
The "central" region of moduli space of M- and string theories is where the
string coupling is about unity and the volume of compact dimensions is about
the string volume. Here we argue that in this region the non-perturbative
potential which is suggested by membrane instanton effects has the correct
scaling and shape to allow for enough slow-roll inflation, and to produce the
correct amplitude of CMB anisotropies. Thus, the well known theoretical
obstacles for achieving viable slow-roll inflation in the framework of
perturbative string theory are overcome. Limited knowledge of some generic
properties of the induced potential is sufficient to determine the simplest
type of consistent inflationary model and its predictions about the spectrum of
cosmic microwave background anisotropies: a red spectrum of scalar
perturbations, and negligible amount of tensor perturbations.Comment: 9 pages, 1 figur
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